Nimh power packs

[Peter Christy]

I must admit to being both baffled and alarmed by this thread! I thought everyone knew from time immemorial that parallel connecting NiXX cells was a big no-no!

Just because it is acceptable practice for LiPos, doesn't mean it will work for other chemistries. A duff or weak cell in a NiXX pack WILL pull down a parallel connected one.

The only way to prevent this is to put a diode in series with each pack, so that should a cell go down, the diode will be reverse biased and won't pull down the good pack. BUT, doing this will cause a 0.7 volt drop across the diode (assuming a silicon type) when forward biased AND will increase the "internal resistance" of the combined pack. This will result in more voltage jitter as the servos switch on and off, and may cause range issues.

Putting physical damage to one side, battery failure will generally occur during charging, and should be easily detected. It is highly unusual for a receiver pack to fail in flight for anything other than physical damage. A pack may lose capacity, but this should normally be detected as part of routine maintenance.

You may get away with parallel connected NiMh cells if both packs are the same age and from the same manufacturer. But they will NOT provide any protection against a cell failure. They *may* provide against a weld or wire failure inside the pack, but that is all. Indeed, by lulling the pilot onto a false sense of security, they could actually be counter-productive! Don't do it!

One of the arguments used by the airline industry for replacing 4-engined jets (747s) with twin engined types is that engine failure is so rare, that halving the number of engines halves the number of possible points of failure. I would suggest that the same argument applies to good quality NiXX battery packs.

Far more likely to fail is the switch harness, and I CAN see a good reason for paralleling two harnesses up in a big model!

--

Pete

[Frank Skilbeck]

Posted by Peter Christy on 01/08/2016 14:15:21:

Far more likely to fail is the switch harness, and I CAN see a good reason for paralleling two harnesses up in a big model!

--

Pete

Or go for one of the electronic switches which fail in their current state, i.e. if on and if the mechanical actuating switch fails then they stay on, plus all the mechanical switch does is activate the electronic switch so it doesn't carry any current.

[Nigel R]

Couldn't agree with Peter more. One good pack, mount it in such a way as to be isolated from vibration as much as possible, and two switches if you must. Simplicity is worth a lot.

"cause a 0.7 volt drop across the diode (assuming a silicon type) "

For what its worth, a well picked shottkey diode will drop far less, 0.2V or thereabouts.

 

". Continuing to charge a NiMH pack at low current is not best practice but will not lead to thermal runaway as far as I've always been lead to believe."

Indefinite trickle charging on nimh is OK at very low rates. Less than C/10 I believe. Which gives at least a 15 hour charge time, by the time charging inefficiencies are done with.

 

 

Edited By SuperNash on 01/08/2016 17:59:22

[Martin Harris - Moderator]

...but this is well within the sort of currents involved in any cross charging.

Peter is quite correct that cell failure will normally happen during charging and not during discharge - a situation I'm very happy with!  I do an individual voltage check on each pack before flying.

Separate leads and switches to a single battery? Fine, but I'd be happier with separate wiring right back to the cell tags.

Edited By Martin Harris on 01/08/2016 18:25:39

[Martin Harris - Moderator]

One thing that has been nagging at me while this discussion has been going on is that the "expert source" I've quoted, Red Scholefield, was referring to NiCd batteries when he wrote his article on parallel operation.

I have to admit that I haven't used NiMHs for this system as I have a great mistrust of them and have used good quality NOS NiCds for many years and am moving on to LiFe cells gradually as my stocks deplete.

I dug out a couple of old NiMHs to do a little practical test - fully charged one and discharged the other to an (on-load of 0.1A) voltage of 3.6v

Oddly, it's off-load voltage recovered to fractionally over 5v while the fully charged on was just under 5.5v after being off charge for a while. These are VERY old packs so may represent failing cells well - I don't have any newer ones to play with.

Connecting them in parallel gave no detectable current flow! I tried right down to a 200uA scale as well as on a known good 200mA one - the batteries were definitely connected together as I got the expected voltage difference when measuring voltage instead of current from the same test point. This appears to bear out Red's statement that "One pack cannot charge the another (equal number of cells) as the discharge voltage of a pack can never be as high as the voltage required to charge the other pack".

I'm still not happy that I couldn't detect even a few uA but I also left the packs connected via a Y lead for quite a while and there was certainly no sign of thermal runaway.

The oft-quoted dreaded scenario of a cell short creating a significant imbalance doesn't seem to me to be terribly relevant - if using a single battery, many receivers will fail to operate correctly on 3 cells (although it is exceptionally unlikely to happen in operation) and my belief is that the resultant voltage with one pack failed in this way on a 2 battery solution would be more likely to allow limited continued operation, negating the doubling of the chance of a random failure.

Bottom line is that the use of parallel batteries is, for me, a reasonable method of decreasing the overall risk of losing control of my model - others are happier with diodes, expensive battery backers, doubled wiring or just the standard single battery and switch and as we can't reasonably eliminate every single point of failure there is always an element of compromise so the decision is ultimately one of personal (hopefully informed) choice.

Edited By Martin Harris on 03/08/2016 14:46:43

[Keith Berriman]

Has anybody recomended that Trebor should ask the question of the seller if amenable why two battery packs installed

[trebor]

If I remember correctly it was a fail safe, just in case one pack dropped. At the time I thought it was a good idea. There are individual switches so I can charge and run separately if I need to and I'm going to put a digital volt display instead of the strip light type.

I have to admit I'm still undecided, it's also a learning curve for me.

[Shaunie]

For what it's worth I have a 68" Edge 540 with a four cell sub-C 3200 Rx pack in it. I've always been concerned about the main point of failure, the on-off switch. I was intending to go to two five cell packs with dual switches and Schottky diodes. I am not worried about diode reliability as I have a considerable stock of 40amp devices ( from when I used to work for International Rectifier). I'm sure a complete short circuit won't hurt those puppies!

Shaunie.

[Martin Harris - Moderator]

40A would certainly make short work (pun unintended but it can stay in!) of the average switch harness wiring - I'd agree that's a perfectly good set-up if you want to use diodes.